Ketyl Radicals Generated from Magnesium(I) Compounds: Useful Reagents for C-C Bond Forming Reactions.

The aryl ketones, 9-fluorenone (fluor), 9-xanthenone (xanth) and 9,10-anthraquinone (anth), were reacted with ß-diketiminato dimagnesium(I) compounds, [{(Ar Nacnac)Mg}2 ] (Ar Nacnac=[HC(MeCNAr)2 ]- , Ar=mesityl (Mes) or 2,6-diisopropylphenyl (Dip)). This gave stable magnesium ketyl complexes which are monomeric, [(Ar Nacnac)(DMAP)Mg(fluor⋅)] (Ar=Mes or Dip, DMAP=4-dimethylaminopyridine) and [(Mes Nacnac)Mg(xanth⋅)(xanth)]; dimeric, [{(Mes Nacnac)Mg(µ-fluor⋅)}2 ], or tetrameric, [{(Dip Nacnac)Mg(µ-anth⋅)}4 ]. In contrast, di-2-pyridylketone (OCPy2 ) is doubly reduced with [{(Xyl Nacnac)Mg}2 ] (Xyl=xylyl) to give a diamagnetic alkoxy/amido complex, [{(Xyl Nacnac)Mg}2 (µ-OCPy2 )]. These complexes have been characterized by X-ray crystallography, and in three cases, EPR spectroscopy. Regioselective C-C hetero-coupling reactions between magnesium ketyls and phenanthroline (phen) have yielded the alkoxy compounds, [(Mes Nacnac)Mg(OCR2 -2-phen)] (OCR2 =xanth, OCPh2 or OC(Ph)(2-Me-Ph)). In addition, homo- C-C couplings of the enones, chalcone (chalc) and dibenziylideneactetone (DBA), using magnesium(I) reducing agents, have afforded dimagnesium enolates, [{(Ar Nacnac)(THF)Mg}2 (µ-chalc2 )] (Ar=Mes or Xyl) and [{(Dip Nacnac)(THF)Mg}2 (µ-DBA2 )]. A pinacol coupling reaction between [{(Dip Nacnac)Mg}2 ] and 2-adamantanone (OAd) yielded [{(Dip Nacnac)(OAd)Mg}2 (µ-OAd2 )], presumably via a ketyl intermediate. This study further highlights the utility magnesium(I) compounds have as selective reducing agents in organic transformations.

C-H Activation of Inert Arenes using a Photochemically Activated Guanidinato-Magnesium(I) Compound.

UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg}2 ] 3 (Priso=[(DipN)2 CNPri 2 ]- , Dip=2,6-diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic C-H bond of the solvent in all cases, and formation of aryl/hydride bridged magnesium(II) products, [{(Priso)Mg}2 (µ-H)(µ-Ar)] 4-9. In contrast to similar reactions reported for ß-diketiminate coordinated counterparts of 3, these C-H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p-NC5 H4 But (pyBut ), gave [(Priso)Mg(pyBut H)(pyBut )2 ] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(pyBut ⋅)(pyBut )2 ] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene C-H activations. One of these involves photochemical cleavage of the Mg-Mg bond of 3, generating magnesium(I) doublet radicals, (Priso)Mg⋅. These then doubly reduce the arene substrate to give "Birch-like" products, which subsequently rearrange via C-H activation of the arene. Circumstantial evidence for the photochemical generation of transient magnesium radical species includes the fact that irradiation of a cyclohexane solution of 3 leads to an intramolecular aliphatic C-H activation process and formation of an alkyl-bridged magnesium(II) species, [{Mg(µ-Priso-H )}2 ] 12. Furthermore, irradiation of a 1 : 1 mixture of 3 and the ß-diketiminato dimagnesium(I) compound, [{(Dip Nacnac)Mg}2 ] (Dip Nacnac=[HC(MeCNDip)2 ]- ), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)Mg-Mg(Dip Nacnac)] 13. Finally, the EPR spectrum (77 K) of a glassed solution of UV irradiated 3 is dominated by a broad featureless signal, indicating the presence of a doublet radical species.

Exploring Equilibria between Aluminium(I) and Aluminium(III): The Formation of Dihydroalanes, Masked Dialumenes and Aluminium(I) Species.

The design of new reductive routes to low oxidation state aluminium (Al) compounds offers the opportunity to better understand redox processes at the metal centre and develop reactivity accordingly. Here, a monomeric AlI compound acts as a stoichiometric reducing agent towards a series of AlIII dihydrides, leading to the formation of new low oxidation state species including symmetric and asymmetric dihydrodialanes, and a masked dialumene. These compounds are formed by a series of equilibrium processes involving AlI , AlII and AlIII species and product formation can be manipulated by fine-tuning the reaction conditions. The transient formation of monomeric AlI compounds is proposed: this is shown to be energetically viable by computational (DFT) investigations and reactivity studies show support for the formation of AlI species. Importantly, despite the potential for the equilibrium mixtures to lead to ill-defined reactivity, controlled reactivity of these low oxidation state species is observed.

Exploring the Gas-Phase Formation and Chemical Reactivity of Highly Reduced M8 L6 Coordination Cages.

Coordination cages with well-defined cavities show great promise in the field of catalysis on account of their unique combination of molecular confinement effects and transition-metal redox chemistry. Here, three coordination cages are reduced from their native 16+ oxidation state to the 2+ state in the gas phase without observable structural degradation. Using this method, the reaction rate constants for each reduction step were determined, with no noticeable differences arising following either the incorporation of a C60 -fullerene guest or alteration of the cage chemical structure. The reactivity of highly reduced cage species toward molecular oxygen is "switched-on" after a threshold number of reduction steps, which is influenced by guest molecules and the structure of cage components. These new experimental approaches provide a unique window to explore the chemistry of highly-reduced cage species that can be modulated by altering their structures and encapsulated guest species.

Synthesis of a Hexameric Magnesium 4-pyridyl Complex with Cyclohexane-like Ring Structure via Reductive C-N Activation.

The reaction of [{(Arnacnac)Mg}2] (Arnacnac = HC{MeC(NAr)}2, Ar = 2,6-diisopropylphenyl, Dip, or 2,6-diethylphenyl, Dep) with 4-dimethylaminopyridine (DMAP) at elevated temperatures afforded the hexameric magnesium 4-pyridyl complex [{(Arnacnac)Mg(4-C5H4N)}6] via reductive cleavage of the DMAP C-N bond. The title compound contains a large s-block organometallic cyclohexane-like ring structure comprising tetrahedral (Arnacnac)Mg nodes and linked by linear 4-pyridyl bridging ligands, and the structure is compared with other ring systems. [(Dipnacnac)Mg(DMAP)(NMe2)] was structurally characterised as a by-product.

Reversible Reductive Elimination in Aluminum(II) Dihydrides.

Oxidative addition and reductive elimination are defining reactions of transition-metal organometallic chemistry. In main-group chemistry, oxidative addition is now well-established but reductive elimination reactions are not yet general in the same way. Herein, we report dihydrodialanes supported by amidophosphine ligands. The ligand serves as a stereochemical reporter for reversible reductive elimination/oxidative addition chemistry involving AlI and AlIII intermediates.

Activation of Ethylene by N-Heterocyclic Carbene Coordinated Magnesium(I) Compounds.

Reactions of a series of magnesium(I) compounds with ethylene, in the presence of an N-heterocyclic carbene (NHC), have been explored. Treating [{(Mes Nacnac)Mg}2 ] (Mes Nacnac=[HC(MeCNMes)2 ]- , Mes=mesityl) with an excess of ethylene in the presence of two equivalents of :C{(MeNCMe)2 } (TMC) leads to the formal reductive coupling of ethylene, and formation of the 1,2-dimagnesiobutane complex, [{(Mes Nacnac)(TMC)Mg}2 (µ-C4 H8 )]. In contrast, when the reaction is repeated in the presence of three equivalents of TMC, a mixture of the ß-diketiminato magnesium ethyl, [(Mes Nacnac)(TMC)MgEt], and the NHC coordinated magnesium diamide, [(Mes Nacnac-H )Mg(TMC)2 ], results. Four related products, [(Ar Nacnac)(TMC)MgEt] (Ar=2,6-dimethylphenyl (Xyl) or 2,6-diisopropylphenyl (Dip)) and [(Ar Nacnac-H )Mg(TMC)2 ] (Ar=Xyl or Dip), were similarly synthesised and crystallographically characterized. Computational studies have been employed to investigate the mechanisms of the two observed reaction types, which appear dependent on the substitution pattern of the magnesium(I) compound, and the stoichiometric equivalents of TMC used in the reactions.

Diagonally Related s- and p-Block Metals Join Forces: Synthesis and Characterization of Complexes with Covalent Beryllium-Aluminum Bonds.

Additions of beryllium-halide bonds in the simple beryllium dihalide adducts, [BeX2 (tmeda)] (X=Br or I, tmeda=N,N,N',N'-tetramethylethylenediamine), across the metal center of a neutral aluminum(I) heterocycle, [:Al(Dip Nacnac)] (Dip Nacnac=[(DipNCMe)2 CH]- , Dip=2,6-diisopropylphenyl), have yielded the first examples of compounds with beryllium-aluminum bonds, [(Dip Nacnac)(X)Al-Be(X)(tmeda)]. For sake of comparison, isostructural Mg-Al and Zn-Al analogues of these complexes, viz. [(Dip Nacnac)(X)Al-M(X)(tmeda)] (M=Mg or Zn, X=I or Br) have been prepared and structurally characterized. DFT calculations reveal all compounds to have high s-character metal-metal bonds, the polarity of which is consistent with the electronegativities of the metals involved. Preliminary reactivity studies of [(Dip Nacnac)(Br)Al-Be(Br)(tmeda)] are reported.

Distinct stepwise reduction of a nickel-nickel-bonded compound containing an α-diimine ligand: from perpendicular to coaxial structures.

A nickel-nickel-bonded complex, [{Ni(µ-L(.-))}2] (1; L=[(2,6-iPr2C6H3)NC(Me)]2), was synthesized from reduction of the LNiBr2 precursor by sodium metal. Further controllable reduction of 1 with 1.0, 2.0 and 3.0 equiv of Na, respectively, afforded the singly, doubly, and triply reduced compounds [Na(DME)3]·[{Ni(µ-L(.-))}2] (2; DME=1,2-dimethoxyethane), [Na(Et2O)]Na[(L(.-))Ni-NiL(2-)] (3), and [Na(Et2O)]2Na[L(2-)Ni-NiL(2-)] (4). Here L represents the neutral ligand, L(.-) denotes its radical monoanion, and L(2-) is the dianion. All of the four compounds feature a short Ni-Ni bond from 2.2957(6) to 2.4649(8) Å. Interestingly, they display two different structures: the perpendicular (1 and 2) and the coaxial (3 and 4) structure, in which the metal-metal bond axis is perpendicular to or collinear with the axes of the α-diimine ligands, respectively. The electronic structures, Ni-Ni bonding nature, and energetic comparisons of the two structure types were investigated by DFT computations.

Synthesis and onwards coordination of an As(I) centered zwitterion.

The bis(phosphino)borate ligand class is used as an anionic anchor to stabilize reactive, low coordinate arsenic centers. The neutral, zwitterionic As(I) species, 2, is formed very cleanly, and isolated in good yields using cyclohexene as a halogen scavenger. The uniqueness of this heterocyclic As(I) compound is on display with the coordination to Group 6 metal centers, (2 M(CO)5; M = Cr, Mo, W). The arsenic-metal bond lengths are longer than the related AsPh3 complexes suggesting that compound 2 is a weak sigma donor. The metal complexes reveal a trigonal pyramidal arsenic atom, which provides the first experimental evidence for the presence of two "lone pairs" of electrons on the As(I) center. When more flexible and more electron-donating isopropyl substituents were used, an intermediate (compound 5) in the formation of low coordinate pnictogen compounds was crystallographically characterized. This structure, formally a base-stabilized dichloroarsenium cation, provides an alternative mechanistic proposal to the one described in the literature.

Quasi-planar Co atom-doped boron cluster: CoB192.

PURPOSE AND METHODS: A global search for the lowest energy structure of CoB192- clusters was conducted.  RESULTS: Its ground state is a quasi-planar structure with the Co atom surrounded by a B8 ring. The central Co atom has an oxidation state of +1 with d8 electron configuration. The wave function analysis showed that the Co-B interaction is not a covalent bond. The bonding strength of peripheral B-B bonds is stronger than that of inner ones. The inner B8 ring bonds with outer boron atoms via σ- and π-type bonds. CONCLUSION: CoB192- shows remarkable aromatic character.

Anchoring an Fe Dimer on Nitrogen-Doped Graphene toward Highly Efficient Electrocatalytic Ammonia Synthesis.

Electrochemical reduction of N2 to NH3 based on sustainable energy is a green technique to produce decentralized and on-demand ammonia. In this work, taking graphene as a design platform, we explore the dual-atom catalysts (DACs) via embedding two homonuclear transition metal (TM) atoms into graphene decorated with four neighboring pyrrolic nitrogen atoms (TM2N4@graphene) to computationally screen the qualified nitrogen reduction reaction (NRR) catalysts. On the basis of the activity, selectivity, and stability of 15 homonuclear DACs of TM2N4@graphene, Fe2N4@graphene is identified as the most efficient NRR catalyst with a limiting potential of only -0.32 V. Electronic structure analysis demonstrates that the low oxidation state of Fe (+1) remarkably activates the molecular N2, which contributes to its excellent NRR catalytic activity. Moreover, the kinetic studies reveal all of the NRR elementary steps exhibiting barriers smaller than that of the hydrogen evolution reaction (HER), showing that HER is effectively suppressed. In addition, we find that the integral crystal orbital Hamilton population (ICOHP) can be used as a descriptor to describe the Gibbs free energy of each step for its NRR performance. This work not only provides theoretical guidance for designing DACs for NRR but also promotes the understanding of DACs for N2 fixation.

Beryllium Halide Complexes Incorporating Neutral or Anionic Ligands: Potential Precursors for Beryllium Chemistry.

Reactions of the beryllium dihalide complexes [BeX2 (OEt2 )2 ] (X=Br or I) with N,N,N',N'-tetramethylethylenediamine (TMEDA), a series of diazabutadienes, or bis(diphenylphosphino)methylene (DPPM) have yielded the chelated complexes, [BeX2 (TMEDA)], [BeX2 {(RN=CH)2 }] (R=tBu, mesityl (Mes), 2,6-diethylphenyl (Dep) or 2,6-diisopropylphenyl (Dip)), and the non-chelated system, [BeI2 (κ1 -P-DPPM)2 ]. Reactions of lithium or potassium salts of a variety of ß-diketiminates have given both three-coordinate complexes, [{HC(RCNAr)2 }BeX] (R=H or Me; Ar=Mes, Dep or Dip; X=Br or I); and four-coordinate systems, [{HC(MeCNPh)2 }BeBr(OEt2 )] and [{HC(MeCNDip)(MeCNC2 H4 NMe2 }BeI]. Alkali metal salts of ketiminate, guanidinate, boryl/phosphinosilyl amide, or terphenyl ligands, lead to dimeric [{BeI{µ-[(OCMe)(DipNCMe)]CH}}2 ], and monomeric [{iPr2 NC(NMes)2 }BeI(OEt2 )], [κ2 -N,P-{(HCNDip)2 B}(PPh2 SiMe2 )NBeI(OEt2 )] and [{C6 H3 Ph2 -2,6}BeBr(OEt2 )], respectively. Compound [{HC(MeCNPh)2 }BeBr(OEt2 )] undergoes a Schlenk redistribution reaction in solution, affording the homoleptic complex, [{HC(MeCNPh)2 }2 Be]. The majority of the prepared complexes have been characterized by X-ray crystallography and multi-nuclear NMR spectroscopy. The structures and stability of the complexes are discussed, as is their potential for use as precursors in poorly developed low oxidation state beryllium chemistry.